The present invention relates to a catalyst mixture and also a method for the production of a polyester melt with high viscosity, the granulate obtained therefrom having an intrinsic viscosity of >0.70 dl/g and an L* colour >70 and the b* colour being between −5 and +5. The catalysts being used during the production are not based on heavy metals but on titanium compounds. Also no components of catalysts based on heavy metal are added. The granulate can be processed further in any way, e.g. to form bottles, containers, films, foils or fibres.
The invention relates to the production in particular of polyethylene terephthalate, subsequently termed PET. PET belongs to the group of polyesters which are characterised by the reaction of a dicarboxylic acid or esters thereof with a diol to form molecules with a long-chain construction. In the case of PET, the dicarboxylic acid is terephthalic acid, subsequently termed TPA, or the ester is dimethylterephthalate and the diol ethylene glycol, subsequently termed EG. At present, the world-wide PET production is approx. 36,000,000 metric tons per year and is used in particular in bottle production, the packaging industry, fibre production and in engineering polymer.
In 1937, Wallace H. Carother applied for a patent for the production of polyesters, in 1949 Whinfield and Dickson for polyesters based on TPA.
The production of polyesters at this time was produced exclusively in the batch method. Only after 1960 were continuous methods introduced, which made production on an industrial scale possible.
All continuous methods consist of the reaction steps of esterification and polycondensation. In the case of esterification, TPA and EG react to form a diester, termed bishydroxyethylterephthalate, in simplified terms BHET. This reaction is self-catalysed by the presence of H+ ions of the reaction partners and therefore requires no further externally supplied catalysts.
In the case of the subsequent polycondensation, essentially the carboxyl- and hydroxyl end groups present react with the emission of EG to form long molecule chains. The reaction speed of this reaction is influenced by increased temperature, dwell time, pressures in the vacuum range, surface renewal rates and very crucially by catalysts.
Whilst in 1949 Whinfield and Dickson still achieved higher molecular PET without the addition of catalysts after 72 hours dwell time, it was in fact soon recognised that catalysts based on antimony and titanium could reduce the dwell time to a few hours.
In the book Polyester Fibres, Chemistry and Technology, of 1975, Ludewig mentions the catalysts known up till then for polymerisation. Thereafter, the various catalysts, usually metal acetate salts, are subdivided into different reactivities. Catalysts, such as antimony-, germanium-, and titanium compounds, are thereby of the highest quality catalytically. Second-class catalysts for the polymerisation reaction are based on elements of the 1st and 2nd main group, in addition aluminium, lead and manganese.
In addition to the reactivity of a catalyst, the selectivity is however also of interest with respect to secondary reactions. In the case of PET, the mainly proceeding secondary reactions produce undesired yellow colourations or increased acetaldehyde- and diethylene glycol generation. With respect to selectivity, germanium and antimony should be mentioned in the first place and both catalysts, in particular the more reasonably priced antimony, have therefore been able to hold their ground over many decades relative to the more reactive titanium compounds.
A further important aspect in the use of catalysts is their complete or partial deactivation since all catalysts also catalyse degradative reactions to some extent, which become noticeable already in production or also only later in further processing or even only in the end product. As deactivation means, more commonly termed stabilisers, phosphorus compounds of all types have thereby proved their worth.
Continuous industrial processes for the production of PET divide the polymerisation to form high-molecular PET into two steps, melt polymerisation and solid-state polymerisation. In the first step, PET is polymerised in a melt with a molecular weight up to approx. 18,000 g/mol, which corresponds to approx. 100 monomer units or an intrinsic viscosity of approx. 0.60 dl/g.
Direct processing to form foils or fibres is effected subsequently or the polymer melt is supplied for granulation in order to obtain defined small PET granulate particles. This granulate is then supplied for solid-state postcondensation (subsequently termed SSP (solid state postcondensation)). Adjusting the molecular weight is effected by the level of the chosen temperature, generally between 210 and 225° C., and the dwell time. In order that the PET chips experience no oxidative damage, nitrogen is taken as carrier gas for heat input and for removal of the resulting reaction products. The level of the final molecular weight depends upon the desired end application. In the case of PET granulates for the production of PET bottles, the molecular weight is approx. 26,000 g/mol, which corresponds to approx. 140 monomer units or an intrinsic viscosity of approx. 0.80 dl/g.
Only since 2007 has there been a continuous industrial process by the company Uhde Inventa-Fischer in which a molecular weight of approx. 26,000 g/mol is already achieved in the melt. Hence the second complex SSP process is dispensed with and only conditioning with air in order to produce for example PET suitable for bottles is required.
In the last 10 to 20 years, great interest has been shown in catalysts for PET which are free of heavy metals, detectable in many discussions in world-wide PET congresses and a large number of patent applications relating to this topic. Not only can greater human and environmental acceptability thereby be detected as driving force but likewise an improvement in PET product qualities. In addition to antimony, also cobalt and bismuth count as heavy metals.
In order to replace antimony as catalyst, there are found in the patent literature all the elements and their compounds or combinations thereof mentioned already for preference in Ludewig, generally in combination with a P component. As varied and specific as all indicated antimony-free formulations are, they all however deal with a formulation for the production of high-molecular PET in the melt polymerisation up to a maximum intrinsic viscosity of approx. 0.60 dl/g and subsequent solid-state postcondensation.
In the state of the art, reference is made very explicitly to the huge difficulties, for highly-viscous polymer melts of approx. 0.80 dl/g, with yellow discolouration and acetaldehyde formation when using catalysts which are not based on antimony.
U.S. Pat. No. 7,368,522 relates to a method with antimony as catalyst, an intrinsic viscosity of at least 0.75 dl/g in the polymer melt (i.e. without SSP) with simultaneously good colours and a short reaction time being intended to be achieved. U.S. Pat. No. 7,368,522 thereby conjectures that this cannot be achieved with titanium formulations.
U.S. Pat. No. 6,559,271 B2 discloses a formulation based on titanium compounds in combination with cobalt which can also be used in addition up to 280° C. In this formulation, cobalt acts both as co-catalyst and as blue colourant in order to control the yellow tone. In order that the acetaldehyde contents can be controlled, the catalysts are deactivated with a P compound and in addition substances which bind acetaldehyde are used. This formulation can be used for higher molecular weights in the melt of 0.63 to 1.00 dl/g intrinsic viscosity but it is not free of heavy metals.
U.S. Pat. No. 7,094,863 B2 claims an antimony-free catalyst formulation for the production of in particular bottle granulate. In particular the improved product properties, such as clarity and dimensional stability for hot filling applications, are thereby highlighted. However, in addition to cobalt, also antimony contents up to 50 ppm are allowed in the formulation. The invention relates to the polyester production by means of SSP. Hence no high viscosities in the polymer melt are achieved with this formulation.
U.S. Pat. No. 7,544,762 B2 describes a formulation which is free of heavy metals and has good colours and low acetaldehyde contents, based on a titanium-phosphorus component. The maximum achieved viscosities of the melt are indicated at 0.64 dl/g.
US 2004/0044173 A1 describes a formulation which is free of heavy metals and has good colours and low acetaldehyde contents, based on a titanium-phosphorus component and the addition of element compounds of the group Ia, IIa, Mg, Fe or Co. The maximum achieved viscosities of the melt are indicated at 0.64 dl/g and require an SSP in order to achieve higher viscosities.
WO 2004/065452 A1 uses a Ti—Na-glycolate as catalyst system and reaches viscosities in the melt of 0.63 to 0.66 dl/g. Here also, an SSP is used to increase the molecular weight.
EP 1 013 692 uses a solid Ti catalyst, obtained by dehydrogenation of a titanium-halogen compound and reaction with a P compound and use of an Mg compound as co-catalyst. As an alternative to the Mg, elements of the IIa group and many heavy metals are mentioned. The achieved viscosities in the melt are at approx. 0.65 dl/g and require an SSP to achieve higher viscosities.
Furthermore, US 2007/010648 A1 relates to the use of a combination of Ti, Zr or Hf with 2-hydroxy-carboxylic acids and a quaternary ammonium compound instead of the normal Ti alkoxides. Here also the achieved viscosities in the melt are only at approx. 0.62 dl/g and require an SSP to achieve higher viscosities.
Qi et al. (EP 2 006 315) likewise use a titanium, phosphorus and co-catalyst system for the production of PET, the co-catalyst preferably being a mixture of Mg, Mn, Ca and Co. With Co, this mixture is also not free of heavy metals. Although also titanium compounds with lactic and citric acid are used, only a viscosity of approx. 0.67 dl/g in the melt is demonstrated with this system.
WO 2008/150350 A1 describes a Ti-based catalyst system with subsequent P addition for the PET production, with which a high viscosity in the melt can be achieved in a reduced reaction time with a low acetaldehyde content. The use of an SSP is thereby regarded as no longer required. The patent describes in addition the use of further additives, inter alia also the use of TiN. In contrast to the described process technology, only simple batch tests are however indicated in the examples. In contrast to commercial reality, extremely high quantities of toner (red 7-9 ppm, blue 13-18 ppm) are used and the colour values achieved are consequently extremely adulterated.
In the case of the catalysts known from the state of the art, it has however always been problematic to date that the obtained products have either sufficiently high viscosity, which led however to problems with discolouration, or had good colour values which led however to problems with viscosity.